U.S. Pat. Nos. 3,983,340 and 4,042,794 to Lima disclose a span line switch which automatically switches pulse code modulated telephone signals from a failed span to a spare span without interrupting the conversation. The switch of these patents was originally designed for use with 1.544 MHz Western Electric Tl spans, and it has been a successful commercial product in that environment for a number of years.
When, however, the switch of the Lima patents was used with high-speed spans such as Western Electric's T1C and T1D, ITT's T148C, and Lenkurt's Modified Duobinary, all of which work on a 3.152 MHz standard, malfunction patterns began to appear. Investigation traced most of these malfunction patterns to the internal resonance characteristics of these spans, and to the resulting increased sensitivity of these high-speed spans to phase changes in the signal and to oscillations induced by repetitive signal patterns.
In addition to having the above-identified operational problems with high-speed spans, the switch of the Lima patents was not usable with ternary systems such as the T148C, or other systems where bipolar violations were not preserved, because the detection of bipolar violations was an integral part of its error detection system.